Within the CDPD network, there is a distinction between two slightly different levels of mobility. Inter-area cell transfer refers to movement of the mobile unit from a cell in one routing area to a different cell in a different routing area. Intra-area cell transfer refers to the movement of a mobile from one cell to another cell, both of which are in the same routing area.
Why is this distinction useful? It has to do with ensuring that the network is optimized for effective use of the most precious resource, airlink bandwidth. Because movement is most often between cells in a local geographic area, it makes sense to optimize this case.
In the CDPD routing architecture described in Section 4.5, we have defined the structure of cells and routing area subdomains. The cells are radio coverage areas which can be small. In fact, within a downtown core region, a mobile user can traverse several cells during a few minutes. This type of movement across cells will likely be occurring often and the system must handle them efficiently.
On the other hand, a routing area subdomain typically spans tens or even hundreds of cells. Mobile unit movement between routing area subdomains should be relatively infrequent. This is especially true with proper network design that encompasses each major traffic corridor within a single routing area subdomain, etc.
Given these concerns about moving M-ESs, the CDPD system has been designed to accommodate fast relocation between cells. Relocation between routing area subdomains requires more administrative interaction, as described in the following section.
In the CDPD network architecture, each routing area subdomain is controlled by a single serving MD-IS, as illustrated in Figure 4.2. In addition, the network protocol architecture, depicted in Figure 4.3, illustrates that the mobile data link is established between the M-ES and the MD-IS. In other words, the MDBS does not participate in the data link connection other than as a relay function.
This means that even as the mobile unit relocates from one cell to another cell controlled by the same MD-IS (illustrated in Figure 4.12), the end-points of the data link connection are not disturbed. Since the data link end-points have not changed even though the cell location has been altered, it is not necessary to disconnect the data link. Therefore, in the CDPD network, this type of mobile relocation can be handled simply with a recognition of a change in the subnetwork point of attachment.
The CDPD system establishes this new subnetwork point of attachment by the detection of traffic for an existing data link connection on a new subnetwork point of attachment. That is, the M-ES announces its movement to a new cell by ensuring that some data link frame is sent in to the network via a channel in the new cell.
If the M-ES relocates to a new cell and immediately has data traffic to transmit, it simply sends the data. The MD-IS, on receipt of data traffic for a data link connection from a new cell recognizes that the M-ES has relocated and updates its registration directory to record the move. If the M-ES relocates to a new cell but has no data traffic to transmit, it transmits a Receiver Ready (RR) MDLP frame. This RR frame triggers the MD-IS to recognize the movement.
This mechanism provides a very efficient method of apprising the network of the mobile's movement. When the mobile unit has data traffic to send, this method does not incur any overhead4.11 . If the mobile unit does not have data traffic to send at the time of the relocation, a very small data frame is transmitted.
Once the serving MD-IS updates its Registration Directory, the process is complete. Since the movement is fully within the scope of control of the serving MD-IS, there is no requirement to inform the home MD-IS of this movement. From this point onward, the serving MD-IS will redirect the data packets destined for that M-ES through the new cell. The protocol events are illustrated in Figure 4.13.
